1. Field of the Invention
The present invention relates to operational amplifiers and more particularly to fully differential current feedback operational amplifiers (CFB op amp's).
2. Background Information
Operational amplifiers have been commonly used for many years, and a particular form of operational amplifiers, current feedback operational amplifiers, have been in use for the last two decades.
Current feedback operational amplifiers have found use in high speed applications such as very fast DAC and ADC (digital to analog and analog to digital converters) and high performance video and audio applications, and the like. CFB op amp's have several circuit characteristics that separate them from standard voltage op amps, such as such a very low input impedance at least at the negative input contact (many CFB op amps have non-symmetrical input characteristics) and bandwidth that is, with some reasonable assumptions, dependent only on the value of the feedback resistance. In typical voltage op amps the bandwidth is dependent on the gain in the closed loop circuit.
A brief review of op amps will be sufficient for those familiar with the art. Voltage op amps have high input impedance, very high voltage gain, an input voltage signal (usually referred to as an error voltage), and a closed loop bandwidth that is dependent on the voltage gain. CFB op amp's have, correspondingly, very low input impedance, very high transimpedance, input current signal (often called an error current), and a closed loop bandwidth that is dependent on the value of the feedback resistor.
The independence of bandwidth and gain allows the gain of a closed feedback loop circuit to be set while largely preserving the bandwidth, as discussed below. This particular feature has prompted designers to use such CFB op amp's in high speed circuit applications where voltage op amps are usually not competitive.
The low input impedance, inter alia, renders CFB op amp's less flexible than their voltage counterparts for many applications, and so they are not as common. Moreover, CFB op amp's have been primarily expressed in bipolar components often due to the larger offset voltages of CMOS components and DC current problems in practical circuits.
For example, consider an application with power rails of ground and +2, and the low input impedance CMOS FET, shown in FIG. 4A, as the front end for an op amp. With practical components, the circuit of FIG. 4A will have a DC voltage level of about +0.5 volts at the low impedance input 20. This is a problem since the output of the op amp will most likely be set to +1 volt to approach a full +/− one volt dynamic output range. In such a case a DC current would flow through the feedback resistor, and this is usually as unacceptable as having the output quiescent voltage be +0.5 V and thereby restricting the output voltage swing.
FIG. 1A shows the familiar voltage op amp equivalent circuit with a very high input impedance, and an error voltage e(err) that is multiplied by a large value B to produce an inverted Vout. The accompanying equation illustrates that Vout is equal to minus R2/R1 at lower frequencies (where LaPlacian term “s” is much lower than 1). The term g(m) is the transconductance of the amplifier.
FIG. 2A shows the basic prior art equivalent circuit for a current feedback op amp, CFB op amp. Here the input impedance is low and the Vout is a function of the error current i(err) times AZm, the transimpedance of the amplifier. In this equivalent circuit v(out)=Av(in), and Av(in)=i(in)Zm, where Zm is the parallel combination of C1 and Rm (representing transresistance). So v(out)/I(in)=AZm.
FIG. 2B shows the typical one pole roll off of AZm (expressed in ohms) with frequency. This is obviously similar to the voltage op amp roll off. Like the voltage op amp gain, AZm is made very large. Also, a capacitor C1 is designed into the circuit to provide this one pole roll off, again to preserve stability. AZm includes the impedance of this capacitor C1 that has, of course, frequency dependent impedance. Importantly, the equation in FIG. 2A shows that the closed loop circuit low frequency gain is −Rf/Ri. As frequency increases (again the s term), the closed loop gain roll off is a function of Rf, the feedback resistor, the capacitor C1, and the open loop gain A, but not Ri. This indicates that the closed loop gain can be made larger by reducing Ri, in FIG. 2A, while Rf remains the same. Indeed, this is true and is well shown in the prior art.
FIG. 2C is a non-inverting circuit using the same current feedback op amp as in FIG. 2A. Here when the absolute value of the transimpedance Azm is large compared to Rf, the voltage gain, V(out)/V(in) is +Rf/Ri.
FIG. 3A is a known high level block that applies to the present invention, however, the present invention's circuit implementation, shown in FIG. 3B, is patently distinct.
The present invention provides a fully differential current feedback amplifier architecture, with a preferred CMOS implementation, but it may be implemented with bipolar or a hybrid circuitry as well. The preferred embodiments below are shown mainly using CMOS, but those skilled in the art will be able to incorporate the present invention in bipolar or hybrid configurations.